1. Field of the Invention
This invention relates to an arrangement in a circulating fluidized bed reactor system for directing exhaust gases from at least one particle separator to a heat recovery section.
2. Description of the Related Art
The circulating fluidized bed reactor system comprises a reaction chamber, having a fluidized bed of solid particles therein and a particle suspension of exhaust gases and solid particles being discharged through at least one discharge opening arranged in the upper part thereof. Each discharge opening is connected to a particle separator for separating the solid particles from the particle suspension. The upper part of each particle separator is provided with a gas discharge opening for the discharge of cleaned exhaust gas flow. The cleaned exhaust gases are directed from the particle separators to a heat recovery section of the circulating fluidized bed reactor system. Each particle separator is connected from its lower part to a return duct, which again is connected to the reaction chamber, for circulating the solid particles separated in the particle separator back to the lower part of the reaction chamber. It is also possible to connect a heat exchanger to the lower part of the return duct for recovering heat from the circulating solid particles.
According to a generally-used manner, the exhaust gases of the particle separators are directed along refractory-lined ductwork to a heat recovery section of the circulating fluidized bed reactor system. This kind of arrangement is disclosed, for example, in the presentation “Development Potentials of Circulating Fluidized Bed Combustion” in VGB report “Thermal Power Plants: The Future of Fluidized Bed Combustion” (1998).
A disadvantage in this kind of arrangement is that erosion and temperature fluctuations cause wearing and embrittlement in the refractory-lined ducts, whereby the ducts require regular maintenance. Furthermore, refractory-lined ducts are heavy and require additional support. Since the ducts have no heat surfaces, it is not possible to recover heat energy from the exhaust gases therein.
The presentation “Large CFB Boiler Plant Design and Operating Experience Texas-New Mexico Power Company 150 MWe (net) CFB Power Plant” published in ASME-conference publication 1995, vol. 2, “Fluidized Bed Combustion,” discloses another arrangement for directing exhaust gases from particle separators to the heat recovery section of a circulating fluidized bed reactor system. The exhaust gases flowing through the gas discharge openings of the particle separators are first directed through discharge ducts to a horizontal extension of the heat recovery section, which is bent above the reaction chamber of the circulating fluidized bed reactor. From there, the exhaust gases are further directed to a vertical part of the heat recovery section.
A significant disadvantage in such a circulating fluidized bed reactor system is that it is difficult to extend vertically running tubes of the vertical part of the heat recovery section to the horizontal part of the heat recovery section. Another disadvantage is the need for a complicated support of the horizontal extension of the heat recovery section and of the reaction chamber.
The presentation “Design Considerations for Circulating Fluidized Bed Steam Generators” published in ASME conference publication 1989, “1989 International Conference on Fluidized Bed Combustion,” discloses an arrangement for directing exhaust gases from two particle separators to the heat recovery section of a circulating fluidized bed reactor system, in which arrangement a gas plenum is positioned above and integrated with a reaction chamber for directing exhaust gases from particle separators to the heat recovery section. Side walls of the gas plenum are formed by water tube panels of walls of the reaction chamber, but the bottom and ceiling of the gas plenum are formed as extensions of water tube panels of the backpass. Such a construction is complicated and may cause stresses due to different thermal expansions.